Field studies have led to several interpretations on the mechanics behind slow earthquake phenomena downdip of the seismogenic zone. To date, field studies have not examined the shallow subduction interface which may also host slow earthquake phenomena. We examine a subduction mélange exhumed from conditions representing the source of shallow slow earthquake phenomena. The mélange consists of a shale matrix containing rigid blocks, including basalt which is altered along the margins. Cataclasite‐bearing faults attest to localized faulting along the altered margins of basaltic blocks, concurrent with distributed shear in the shale matrix. These cataclasite‐bearing faults link individual blocks. Microstructures show mutually crosscutting tensile and shear veins, consistent with failure having occurred at, or near, lithostatic pore fluid pressures. We model the stress concentrations around the altered margins of basaltic blocks during distributed shear and show that frictional failure of the altered basalt is predicted to occur at lower imposed strain rates than frictional failure of the shale, favoring fault development along block margins. Calculations of critical nucleation lengths for the blocks show they would fail dynamically at hydrostatic pore fluid pressures, producing microearthquakes. At near‐lithostatic pore fluid pressures, block lengths are below the critical nucleation length for dynamic failure and may produce slow earthquake phenomena. Mixing of velocity‐weakening blocks into a viscously flowing, velocity‐strengthening matrix may serve as a common mechanism for slow earthquake phenomena updip and downdip of the seismogenic zone.

中文翻译：

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地震成因带上倾极限沿构造混杂岩中玄武岩块体局部破坏的证据：对浅慢地震源的影响

现场研究对慢地震现象在震源带下倾后的力学有多种解释。迄今为止，野外研究还没有研究浅层俯冲界面，这也可能导致缓慢的地震现象。我们研究了从代表浅层慢地震现象发生的条件中挖掘出的俯冲混杂岩。混杂岩由含刚性块的页岩基质组成，包括沿边缘改变的玄武岩。含ac石的断层表现出沿玄武岩块边缘的局部断层，并伴有页岩基质中的分布剪切力。这些含白云母的断层将各个区块联系起来。显微组织显示出相互交叉的拉伸和剪切静脉，这与在岩石静力学孔隙流体压力处或附近发生的破坏一致。我们对分布剪切过程中玄武岩块边缘变化周围的应力集中进行了建模，结果表明，与玄武岩页岩的摩擦破坏相比，改变后的玄武岩的摩擦破坏预计发生在较低的施加应变速率下，有利于沿区块边缘的断层发育。块的关键成核长度的计算表明，它们在静水孔隙流体压力下会动态失效，从而产生微地震。在接近静态的孔隙流体压力下，块体长度低于动态破坏的临界成核长度，并可能产生缓慢的地震现象。将速度弱化块混入粘性流动的速度增强矩阵中，可能是造成地震发生区慢地震现象上倾和下倾的常见机制。